We will investigate the enzymatic properties, origin, and biological function(s) of the core-associated 32K protein containing endonuclease activity that we have isolated from avian myeloblastosis virus and Rous sarcoma virus. (2) The multiple RNase H species associated with mammalian RNA tumor virions will be further characterized. (3) Attempts will be made to isolate and characterize an "early reverse transcriptase complex" from the cytoplasm of cells early after infection with the murine leukemia virus. (4) An enzyme activity that cleaves murine oncornavirus subunit RNA will be further characterized; the in vitro cleavage products will be analyzed and compared with the 20S RNA species isolated from virus producing cells. (5) The intracellular site of synthesis of murine leukemia virus glycoprotein and major core protein will be studied. (6) Non-virion proteins induced by the Moloney sarcoma virus will be identified, isolated, and characterized.